The invention relates to a device for variably setting the control times of gas exchange valves of an internal combustion engine according to the preambles of claims 1 and 2.
In internal combustion engines, camshafts are used to operate the gas exchange valves. Camshafts are arranged in the internal combustion engine such that the cams arranged on them contact the cam followers, such as for example flat-based tappets, finger levers, and rocker arms. When a camshaft is made to rotate, the cams roll over the cam followers, which in turn operate the gas exchange valves. Therefore, both the duration of the opening as well as the opening amplitude and the opening and closing times of the gas exchange valves are determined by the position and the shape of the cams.
Modern motor concepts tend to design the valve drive in a variable fashion. On the one hand, the valve stroke and the duration of the valve opening shall be designed variably, up to completely shutting off individual cylinders. For this purpose, concepts are provided, such as for example cam followers or electro-hydraulic or electric valve actuators that can be switched. Furthermore, it has proven advantageous when the opening and closing times of the gas exchange valves can be influenced during the operation of the internal combustion engine. Here, it is particularly desired when the opening and/or closing times of the intake and/or exhaust valves can be influenced separately, in order to adjust a defined valve overlap in a targeted manner, for example. By adjusting the opening and/or closing times of the gas exchange valves depending on the actual range of the ignition map of the engine, for example the actual rotation and/or the actual load, the specific fuel consumption can be lowered, the exhaust behavior can be influenced beneficially, and the effectiveness of the engine, the maximum torque, and the maximum output can be increased.
The variability of the valve control times described is achieved by a relative change of the camshaft phasing in reference to the crankshaft. Here, the camshaft is usually connected to the crankshaft in a driving fashion via chain, belt, toothed wheel, or drive concepts operating in the same manner. A device for changing the control times of the internal combustion engine is arranged between the chain, belt, or toothed wheel drive driven by the crankshaft and the camshaft, in the following also called a camshaft adjuster, which transfers the torque from the crankshaft to the camshaft. Here, this device is embodied such that during the operation of the internal combustion engine, the phasing of the camshaft in reference to the crankshaft is held securely and, if desired, the camshaft can be rotated in reference to the crankshaft within a certain angular range.
In internal combustion engines provided with one camshaft for each of the intake and the exhaust valves, the valves can be separately provided with one camshaft adjuster. This way the opening and closing times of the intake and exhaust valves can be temporarily shifted in reference to each other and valve overlaps can be adjusted in a targeted manner.
The location of modern camshaft adjusters is usually at the driving end of the camshaft. The camshaft adjuster may also be arranged on an intermediate shaft, a non-rotating component, or the crankshaft. It comprises a driving wheel, driven by the crankshaft and being in a fixed phasing in reference thereto, an output part being in a driving connection with the camshaft, and an adjustment mechanism transferring the torque from the drive wheel to the output part. In case the camshaft adjuster is not arranged at the crankshaft, the drive wheel can be embodied as a chain, belt, or toothed wheel and driven by the crankshaft via a chain, belt, or a toothed wheel drive. The adjustment mechanism can be operated electrically (by a driven three-stage planetary gear), hydraulically, or pneumatically.
The so-called axial piston adjusters and rotary piston adjusters represent two preferred embodiments of hydraulically adjustable camshaft adjusters.
In axial piston adjusters the drive wheel contacts a piston and said piston the output part, each via helical gearing. The piston separates a hollow space, formed by the output part and the drive wheel, into two pressure chambers arranged axially in reference to each other. If now one pressure chamber is impinged with a pressure medium while the other pressure chamber is connected to a reservoir the piston is displaced in the axial direction. By the helical gearing the axial displacement of the piston is converted into a relative rotation of the drive wheel in reference to the output part and thus of the camshaft in reference to the crankshaft.
A second embodiment of hydraulic camshaft adjusters is represented by the so-called rotary piston adjusters. Here, the drive wheel is connected to a stator in a fixed manner. The stator and a rotor are arranged concentrically in reference to each other, with the rotor being connected to a camshaft, an extension of the camshaft, or an intermediate shaft in a force, form, or material-fitting manner, for example via an interference fit, a screwed, or a welded connection. In the stator, several hollow spaces are formed, spaced apart in the circumferential direction, which extend radially outward starting at the rotor. The hollow spaces are bounded in a pressure-tight manner in the axial direction by side caps. A blade, connected to the rotor, extends into each of these hollow spaces dividing each hollow space into two pressure chambers. By a targeted connection of the individual pressure chambers to a pressure medium pump, and/or a reservoir the phasing of the camshaft in reference to the crankshaft can be adjusted and/or upheld.
Sensors detect the characteristics of the engine, such as for example the load condition and the rpm's in order to control the camshaft adjuster. These characteristics are fed to an electronic control unit, which controls the inlet and outlet of pressure medium to the different pressure chambers after a comparison with data (saved) in a data sheet of the internal combustion engine.
In order to adjust the camshaft phasing in reference to the crankshaft in hydraulic camshaft adjusters one of the two reciprocally operating pressure chambers of a hollow space is connected to a pressure medium pump and the other one to a reservoir. The supply of pressure medium to one chamber combined with the outlet of pressure medium from the other chamber displaces the piston separating the pressure chambers in the axial direction, so that in axial piston adjusters, the camshaft is rotated in reference to the crankshaft via the helical gearing. In rotary piston adjusters a displacement of the blade is affected by the impingement of one chamber with pressure and pressure release of the other chamber and thus directly rotates the camshaft in reference to the crankshaft. In order to uphold the phasing both pressure chambers are either connected to a pressure medium pump or both of them are separated from the pressure medium pump and the reservoir.
The control of the flow of pressure medium and/or the pressure chambers occurs via a control valve, usually a 4/3 proportional valve. Each valve housing is provided with one connector for the pressure chambers (operating connector), a connector for the pressure medium pump, and at least one connector to a reservoir. An axially displaceable control piston is arranged within the valve housing essentially embodied in a hollow cylindrical fashion. The control piston can be brought into any axial position between two defined end positions via an electro-magnetic actuator counteracting the spring force of a spring element. The control piston is further provided with circular grooves and control edges, by which the individual pressure chambers can be optionally connected to the pressure medium pump or the reservoir. Similarly, an adjustment of the control piston can be provided, in which the pressure medium chambers are separated both from the pressure medium pump as well as the pressure medium reservoir.
During the starting phase of the internal combustion engine, the camshaft adjusters need a certain amount of time until the phasing can be held securely. In case of a hydraulic camshaft adjuster this is caused in that during the off state of the internal combustion engine, the pressure medium exits the pressure chambers and thus during the start of the internal combustion engine the hydraulic clamping of the piston and/or the blade is not ensured. The camshaft phasing in reference to the crankshaft is not fixed until the oil pump of the internal combustion engine, driven by the camshaft sufficiently provides the camshaft adjuster with pressure medium. Thus, poor start and operating characteristics of the internal combustion engine result. Furthermore, the piston or the blades inside the pressure chambers can be adjusted in an unlimited manner due to the reaction moments of the camshaft, which cause them to hit stops in the device, resulting in noise and causing wear.
This is counteracted such that a rotational angle limiting device is provided, which mechanically couples the output element to the input element and thus prevent a rotation of the two components in reference to each other. Such rotational angle limiting devices are realized by a locking piston, which is arranged in a receiver embodied either in the input element or the output element. Furthermore, a spring is provided, which urges the locking piston into the direction of the other component. Furthermore, a link is provided at the other component, into which the locking piston is pushed when a predetermined locking phase is reached.
Here, it can be advantageous to lock the output element in reference to the input element in one of the two extreme phase positions or in a phase position therebetween. Depending on the application, one or more locking devices are provided, in which the link in the second case can be embodied as a blind hole or a groove extending in the circumferential direction.
In DE 698 17 413 T2 such a device is shown. It relates to a device in a rotary piston design. An input element, being in driven connection with a crankshaft, is supported in a rotary fashion on an output element, connected to a camshaft in a non-rotatable manner. The input element is embodied with recesses open towards the output element. In the axial direction of the device, side caps are provided limiting the device. The recesses are sealed in a pressure tight manner by the input element, the output element, and the side caps and thus form pressure chambers. In the exterior casing surface of the output element, axial grooves are inserted, in which blades are arranged extending into the recesses. The blades are embodied such that they divide the pressure chambers into two reciprocally operating pressure chambers. By supply and/or withdrawal of pressure medium to and/or from the pressure chambers the camshaft phasing in reference to the crankshaft can be held or adjusted.
A locking piston is arranged in the input element, which is impinged by a force of a spring in the direction of the output element. A blind hole is provided at an external casing surface of the output element facing the locking piston. The blind hole is arranged and embodied such that, in a defined phase of the output element in reference to the input element, the locking piston engages the blind hole, when not impinged with the pressure medium. The rotor is therefore locked in reference to the stator, thus preventing a relative rotation. Pressure medium is supplied to the blind hole via a control line, by which a face of the locking piston is impinged with pressure medium. In this manner, the piston is pressed into the receiver and a rotation of the rotor in reference to the stator is allowed in one direction. The control line is provided separate from a device for supplying pressure medium, impinging the pressure chambers with pressure medium. Furthermore, a switch-over valve is provided, which controls the supply and/or withdrawal of pressure medium of the control line. This control valve is switched via a microprocessor and an electromagnetic control unit from a position, in which the pressure medium is withdrawn from the blind hole, into a condition, in which the blind hole is impinged with pressure medium.
Here, the high expenses are disadvantageous arising from the separate control line being operated via an electromagnetic control unit. In order to loosen the lock of the output element in reference to the input element only when the device is filled sufficiently with pressure medium this state must be detected or a certain period of time after the start of the internal combustion engine must be awaited before the switch-over valve is operated. In the first case, sensors must be provided, which must be supervised by the ECU of the internal combustion engine, which leads to higher costs and an increased control expense. In the second case the locking can be released before the desired fill state of the device is reached, which leads to the above-mentioned disadvantages.